Photoinduced bending and unbending behavior of liquid-crystalline elastomers is discussed. Various modes of bending have been achieved with various alignments of the photoactive mesogens in the elastomers: oriented Liquid-crystalline elastomer films were found to undergo anisotropic bending and unbending behavior only along the rubbing direction, when exposed to alternate irradiation of unpolarized UV and visible light; in the case of polydomain Liquid-crystalline elastomer films, a single film could be bent repeatedly and precisely along any chosen direction by using linearly polarized light.

The effect of the introduction of micro-domains in polymer films on the photorefractive effect was investigated. The photorefractivity of side-chain liquid crystalline polymers that possess a hydrogen-bonding moiety was examined using a four-wave mixing experiment. The photorefractivity was compared to that of a polymer which has no hydrogen-bonding moiety. A significant difference was found in the photorefractivity between hydrogen-bonding polymers and non-hydrogen-bonding polymers. This difference was likely due to the presence of micro-domain structures in the polymers with the hydrogen-bonding moieties.

We present results on DC field and light-induced photorefractive-like gratings in liquid crystal cells made with photosensitive polymer layers. Surface charge layers that develop on a liquid crystal-polymer interface are responsible for screening the liquid crystal bulk from an external DC field. These layers can be selectively discharged via polymer photoconductivity and lead to liquid crystal reorientation grating. Efficient two-beam coupling gain can arise from the reorientation grating for relatively high DC field (above 30 V). For lower voltages, strong diffraction can be observed that reduces the gain. Two-beam coupling gain and diffraction can also be controlled by experimental geometry and liquid crystal alignment. The simulated profile of electric field shows that its penetration depth depends on grating spacing.

Dynamic optical reconstruction of digital binary holograms projected on optically addressed liquid crystal spatial light modulators with the use of computer driven multimedia projector is described. The high spatial resolution, sensitivity and full reversibility of manufactured spatial light modulators is achieved by the use of photoconducting PVK:TNF polymer layer serving as a transducer of incoming light intensity pattern into modulation of refractive index inside adjacent LC layer. Linearly polarized laser light reconstructs the phase holograms at the video-rate. The advantages and drawbacks of the presented system are discussed.

We have performed quantitative theoretical and experimental studies of all-optical self-induced polarization switching in thin films and waveguides of nematic liquid crystals, and demonstrated the feasibility of such processes over the entire visible-near infrared [0.4 - 1.55 microns] spectrum. We have also studied the detailed dynamics of the nonlinear optical interaction with the nematic crystalline axis, and observed interesting frequency selective beam amplification and stimulated scattering effects.

Optically pumped laser action has been observed at the edge wavelength of the stop band in a dye-doped flexible freestanding film of photopolymerized cholesteric liquid crystal (PCLC), which originates from band edge effect of the one-dimensional photonic band gap. On the other hand, defect mode laser action has also been experimentally demonstrated at the defect mode wavelength within the band gap. This laser action is based on the photon localization at the twist defect of the composite film consisting of two PCLC layers. Twist defect mode (TDM) is induced by the introduction of twist defect which is a discontinuity of the director rotation around the helix axis at an interface of two PCLC layers. We also propose a new type of tunable defect mode based on the chiral defect in which the partial modulation of the helix pitch is introduced.

Liquid crystal (LC) materials with negative dielectric anisotropy have been successfully applied to light emitting diode. Even for a nematic phase, the Fredericks transition was avoided under such a strong electric field as used in organic light emitting diode (OLED). The LC materials having shallower highest occupied molecular orbital (HOMO) levels have been emissive due to their higher probability for hole injection. The LC materials incorporated three fluorescent dyes for each color (R,G,B) show corresponding color emission. However, only blue dye shows extraordinarily small light emission. Relative fluorescence quantum yield measurement reveals similar fluorescence efficiency for all dyes. Only blue dye’s HOMO level lies very close to that of the matrix LC. This fact may lead to very small hole trapping probability for the blue dye molecule. It is suggested that in the OLED system using LC materials studied here, the recombination of the electrons and the holes trapped at the dye molecules is dominant for light emission rather than Foerster energy transfer.

We have studied the lasing characteristics from a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (PCLC) films as photonic band gap materials. The nematic layer possessing birefringence brings about the following remarkable optical characteristics; (1) reflectance in the photonic band gap (PBG) region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film, (2) efficient lasing occurs either at the notch of PBG or at the photonic band edge, (3) the lasing emisions contain both right- and left-circular polarizations, and (4) tunable lasing can be achieved by the reorientation of nematic liquid crystal molecule under the application of an electric field.

Conjugated organic molecules and polymers are of great interest for applications as the active element in new types of electroluminescent, photovoltaic, and electronic devices. Control of the molecular order of these materials can have a significant impact on their optical and electronic properties. In particular, alignment of conjugated organics that exhibit an intrinsic anisotropy in a dipole moment permits generation of thin films that emit linearly polarized light in an orientation specific to the alignment direction of the molecule. Polarized photoluminescence and electroluminescence from ordered conjugated oligomers and polymers has been demonstrated using many alignment methods, but particular success in integrating these materials into electrical devices has been achieved by utilizing conjugated materials that exhibit a glassy nematic liquid-crystalline phase. We report the status of our ongoing development of OLED devices based on glassy nematic oligofluorenes. These devices exhibit electroluminescent peak polarization ratios as high as 31:1 and color coordinates spanning the entire visible spectrum.

We present the implementation of dynamic Diffractive Optical Elements on three different types of commercially available liquid crystal spatial light modulators, each of them featuring a different modulation capability. The one using Twisted-Nematic liquid crystal exhibits coupled amplitude and phase modulation, the one using analog Ferroelectric liquid crystal a pure amplitude modulation and the one using Nematic liquid crystal a pure phase modulation. Based on experimental results, the performance of these three devices is compared.

It is shown that 2,6 azo-substituted anthraquinone dye-doped systems are interesting alternative to Methyl Red (MR) doped NLCs as materials for Optically Addressed Spatial Light Modulators (OASLMs) without amorphous silicon layer. Nonlinearity in liquid crystals doped with new dye is studied. Dynamic holographic grating formation is observed under conditions of low power laser light and no external fields. The samples are planar and normal incidence of light is used. The results for dynamic holographic studies are compared with azo dye MR crystals and C60. It is shown that this dopant competes with the best known materials in terms of performance in 10 ms speed regime. The system under investigation possesses very good time stability and outstanding light fastness (even a power exceeding working light intensity 100 times is not destructive to the material). It does not form permanent component at any conditions, which is vital for applications where constant change of written information is required (OASLMs, dynamic holography, all-optical switching). Possible mechanisms and the nature of effects that lead to the photorefractive effect in the anthraquinone system are discussed. Resolution of the devices, their efficiency and optimal working conditions are investigated.

The effects of positive nitrogen pressure (0psi to 10psi above ambient) on the reflection spectrum of holographic polymer dispersed liquid crystal (H-PDLC) Bragg gratings has been experimentally analyzed showing shifts in Bragg wavelength and peak broadening effects. We have observed a spectral blue-shift in the peak reflection wavelength that occurs with increasing pressure. Using statistical analysis techniques, we have shown that this is a real effect that can be quantified. The spectral dependence on applied pressure is explained by mechanical deformations to the polymer network, which is assumed to behave linearly. These devices have potential usage as optical pressure sensors.

Integrated optic devices using liquid crystals embedded in optical waveguiding structures have advantages in terms of compactness and high performance. Such devices exploits the high electro-optic effect and the good optical properties of liquid crystals, in particular their high birefringence and their low absorption, combined with low loss optical waveguides. Optical switches based on ion-exchanged channel glass waveguides and liquid crystals operating in the C-band used for optical communications have been designed by using beam propagation method. Polarization independent configurations are described and evaluated in terms of crosstalk and losses. Feasibility of tunable optical filters using composites materials made of polymer and nematic liquid crystals to be used in optical communication and sensor systems is demonstrated. Materials and fabrication procedures of both integrated optical switches and filters are also discussed.

An adaptive lens, which has variable focus and is rapidly controllable with simple low-power electronics, has numerous applications in optical telecommunications devices, 3D display systems, miniature cameras and adaptive optics. The University of Durham is developing a range of adaptive liquid crystal lenses, and here we describe work on construction of modal liquid crystal lenses. This type of lens was first described by Naumov [1] and further developed by others [2-4]. In this system, a spatially varying and circularly symmetric voltage profile can be generated across a liquid-crystal cell, generating a lens-like refractive index profile. Such devices are simple in design, and do not require a pixellated structure. The shape and focussing power of the lens can be controlled by the variation of applied electric field and frequency. Results show adaptive lenses operating at optical wavelengths with continuously variable focal lengths from infinity to 70 cm. Switching speeds are of the order of 1 second between focal positions. Manufacturing methods of our adaptive lenses are presented, together with the latest results to the performance of these devices.

The nature of the liquid crystal (LC) alignment in devices is a crucial parameter in the operating efficiency of these devices and there are numerous methods to induce LC alignment. Nanopatterned substrates of polymers including PMMA and PTFE are known to influence the LC alignment due to the confinement. Nanopatterned polymer substrates created using the NanoIndenter have dimensions 500nm x 500nm x 100nm. Following this procedure, the LC alignment is studied after its deposition these nanopatterned surfaces. The LC alignment variation is observed using a Polarizing Optical Microscope (POM). The POM images will be analyzed to map the pixel values to LC alignment in different patterns like grid structures and grooves. Also, finite difference modeling is used to study the theoretical nature of the LC alignment. The Simulated Annealing algorithm is used to minimize the energy of the LC on the patterned surface. Various boundary conditions like anchoring on the side and top walls and surface anchoring strengths will be applied to observe the nature of the resulting alignment. Modeling results of the LC alignment, when performed on different patterns like square grids, triangular and sawtooth shaped substrates shall be compared with the experimental results.

We have fabricated 1-, 2- and 3-D photonic crystalline structures in polymer dispersed nematic and isotropic phase liquid crystals. It is observed that a particular mixture of isotropic liquid crystals and the photo-polymer will also polymerize and phase separate, forming high quality optical gratings, just as typical nematic liquid crystals. Liquid crystal droplet sizes obtained could be as small as a few 10’s of nm, i.e., nano-droplets. The resultant structure exhibits excellent optical qualities, and high efficiency Bragg diffraction properties.

Photoresponsive liquid crystal physical gels are formed from a hydrogen-bonded gelator containing photochromic azobenzene moieties and nematic or discotic liquid crystals. The bistable gel structures based on the trans-azobenzene gelator could be achieved by combining the trans-cis photoisomerization of the azobenzene moieties and thermal treat-ment. Upon UV irradiation, the trans-cis photoisomerization causes the transition from the initial gel states to the liquid crystal sol states. The cis-trans back-isomerization causes reaggregation of the trans-gelator in the liquid crystals. This leads to the formation of the second gel states which have the structures reflecting the liquid crystal order. The initial gel states can be reversibly changed to the reformed gel states by photoirradiation and thermal treatments. The photo-induced reversible structural changes of the anisotropic physical gels are applied to rewritable information recordings.

When flakes of polymer cholesteric liquid crystals (PCLC's) are dispersed in a fluid host and subjected to an applied electric field, their bright, polarization-selective reflection color is extinguished as they undergo field-induced rotation. Maxwell-Wagner (interfacial) polarization is the underlying physical mechanism for flake motion and results from the large difference in dielectric properties of the flake and fluid hosts. Flake reorientation times can be as short as 300 ms to 400 ms at exceedingly low driving fields (10 to 100 mVrms/μm) and are dependent on flake size and shape, fluid host dielectric constant and viscosity, and drive-filed frequency and magnitude. These attributes make this new materials system of special interest in electro-optical and photonics applications, where reflective-mode operation, polarization selectivity, and low power consumption are of critical importance (e.g., reflective displays).
Until very recently, the electro-optical reorientation of PCLC flakes has been studied only in sandwich-type cells using glass substrates. In this work, we report on the dc field-induced reorientation behavior of PCLC flakes contained in confined spherical or near-spherical fluid-filled cavities formed by microencapsulation of the flake/fluid host dispersion in a water-borne flexible binder. This PCLC flake-fluid host/binder emulsion is coated onto either rigid or flexible condutive-coated substrates and then overcaoted (uniformly or patterned) using a conductive emulsion or paint that is either absorbing (black) or reflecting (silver). In addition to providing a unique environment to study flake motion, this device geometry also extends the application scope of the technology to conformal, electrically switchable coatings for large planar areas and flexible media for information display applications (e.g., electronic paper).

Key challenges in achieving higher-quality liquid crystal displays for future generations are developing new liquid crystal materials with faster electro-optical response and simplifying the fabrication process of devices. Blue phases1-4, kinds of liquid crystal phases, have two major advantages over commonly used nematic phases. First, the response is much faster5-8. Second, the zero-electric field state is optically isotropic unlike nematic phases, that is, no surface treatment (no rubbing) is necessary, leading to a simplification of the fabrication process. One of the problems of blue phases that the available temperature range is very narrow has been overcome by our recent achievement, the polymer-stabilized blue phases9,10. Here we show that the polymer-stabilized blue phases surmount another problem that the blue phase is very fragile to an electric field. We also demonstrate the sufficiently large electric field-induced birefringence and the micro-second response of the polymer-stabilized blue phases without any surface treatment.

We describe a distinct polar nematic liquid crystal formed from the polar rod-like aromatic polyester which comprises 4-hydroxybenzoaic acid (HBA) and 6-hydroxy-2naphthoic acid (HNA) in a molar ratio of 73/27. The nemtic liquid crystal is biaxial and the polarity appears along both axes as determined by measurements of the second harmonic generation. The polar structure disappeasrs when the molecular weight in polyester is decreased, showing that the large dipole moment of each chain is responsible for the polar ordering. The strong dipole-dipole interaction between polar rod-like molecules may be ascribed to the origin of the polarility.

An optically active liquid crystal compound, bis-[4’-(1-methylheptyloxy-carbonyl)-4-biphenyl] terephthalate, possessing two chiral centers at both peripheral ends was prepared, and the liquid-crystalline properties investigated. This compound showed a liquid crystal phase with a 3D superstructure of the defects, i.e., the smectic Q (SmQ) phase, between the antiferroelectric and isotropic liquid phases. Complicated x-ray diffraction spots appeared in the small angle region in the SmQ phase due to the formation of the 3D network of the defects, however, only broad scattering was observed in the wide angle region. Reducing the number of phenyl rings of this compound decreased the stability of the SmQ phase, thus the resulting compound just exhibited the antiferroelectric phase. Even in the isotropic phase above the SmQ or antiferroelectric phase of these compounds, a clear x-ray diffraction scattering was detected in the small angle region, suggesting a possible molecular pre-organization in the isotropic phase. Contact studies showed that another liquid crystal superstructure, i.e., a twist grain boundary phase, was induced by mixing these chiral compounds or by mixing the antiferroelectric compound with an achiral compound. Helical structures induced in the nematic phase were also examined for these and the related chiral compounds.

Electro-optical properties of polymer dispersed liquid crystal (PDLC) or holographic polymer dispersed liquid crystal (H-PDLC) are very sensitive to the photoinduced phase separation process (PIPS). In order to improve initial mixture and recording setup, real time monitoring of diffraction efficiency is currently performed using
a diffusion model based on the moderation Fick's law. Nevertheless, this model does not take into account neither change of affinity for liquid crystal molecules when the monomer polymerization occurs nor the droplets morphology observed by scanning electron microscopy. In this paper, a new model consistent with the general Onsager theory of transport is introduced. As an application, droplet's growth and spatial response of H-PDLC films are described using dimensionless numbers and very general normalized parameters which open new method
of improvement for electro-optical devices based on PDLC's or H-PDLC's.

Electrically tunable laser action has been demonstrated in a dye-doped nematic liquid crystal (NLC) waveguide by holographic excitation. The optical feedback were provided by a transient grating induced by two-beam interference using Lloyd mirror configuration, and the distributed feedback (DFB) laser action was observed. Electrical tuning of lasing wavelength was realized due to the change of the effective refractive index of the NLC core layer caused by the reorientation of NLC molecules. The total shift of lasing wavelength was about 30 nm, which could be realized with less than about 1.4 V of applied voltage. Based on a waveguiding mode theory, numerical analysis of TM-guided mode in the presence of applied electric field was performed, and field-induced tuning of the lasing wavelength was investigated in detail. Prospects for the realization of a single-mode operation and tuning of the lasing wavelength was also shown. Based on the numerical results, single-mode operation of lasing was experimentally realized utilizing NLC with low refractive indices.